Multiple mode excitation apparatus



Aug. 21, 1956 w. w. SALlSBURY MULTIPLE MODE -'EXCITATION APPARATUS Filed Dec. 22, 1950 OSCILLA'IUR INVENTOR. MA/F/ELO M SAL/satiny United States Patent MULTIPLE MODE EXCITATION APPARATUS Winfield W. Salisbury, Cedar Rapids, Iowa, assignor to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application December 22, 1950, Serial No. 202,275

3 Claims. (Cl. 3155) This invention relates in general to linear accelerators and in particular to a multiple mode. exciting system.

Linear accelerators are used for accelerating charged particles to high velocities in order that a large amount of energy may be stored in them. This energy might be used, for example, to bombard a target and thus release X-rays. For a more complete description of linear accelerators, reference may be had to the patent to Sloan, Number 2,398,162, April 9, 1946. Previously excitation of the electron beam traveling through the accelerator has been such as to obtain one-half wave length or some multiple thereof between adjacent fingers.

In a linear accelerator the following relationship exists:

2 Z wherein Z is equal to the shunt impedance, PR, equals the power dissipated in the accelerator, and V is equal to the voltage imparted to the charged particles.

It is to be noted that if the power remains constant, and Z is increased, V will increase. This means that the efiiciency of operation goes up as Z is increased. The shunt impedance may be increased by operating the accelerator in a multiphase mode so that less than 180 degrees phase shift occurs between adjoining fingers.

It is an object of this invention, therefore, to operate a linear accelerator with modes less than 180 degrees between adjoining fingers so as to thus increase the energy imparted to the accelerated particles.

A further object of this invention is to provide a linear accelerator which has a multi-phase feeding arrangement so that excitation may be obtained which has less than 180 degrees between adjoining fingers.

Yet another object of this invention is to provide a more efficient accelerator.

To provide a linear accelerator which has a plurality of radially extending finger-s that are connected at different locations about the accelerating tube so as to decrease the capacity effect between adjoining fingers is another object of this invention.

.A feature of this invention is found in the provision for a linear accelerator which has a plurality of radially inwardly extending fingers that are mounted at various positions about the accelerator cover and which has a multiphase exciter so as to obtain less than 180 degrees phase difference between adjoining fingers.

Further features, objects, and advantages of this invention will become apparent from the following description and claims when read in view of the drawings, in which:

Figure l is a sectional view of the linear accelerator of this invention showing the fingers and their relative spatial relationships;

Figure 2 is a sectional view taken on line 2-2 of Figure 1; and

Figure 3 is a schematic of a means for obtaining multiphase feeding.

Figure 1 illustrates a cylindrical envelope designated generally as which has mounted in one end thereof Patented Aug. 21, 1956.

an electron gun 11. The gun 11 has a filament 12 which is heated with a suitable voltage to an electron emission temperature and an opening 13 is formed in oneend of the gun to allow electrons to emerge therefrom. At the opposite end of the envelope 10 is placed a target 14. A high direct current potential is connected between the target 14 and the electron gun 11 so as to draw the electrons longitudinally through the accelerator.

Connected to the inside wall of the envelope 10 are a plurality of fingers 16 which extend radially past the center of the envelope and which are formed with openings 17 that are axially aligned with the electron gun and the target 14. The fingers 16 are attached to the surface of the envelope 10 at different angular positions so as to decrease the capacity between adjoining fingers. For example, as shown in Figure 1, each finger is rotated degrees from the preceding finger. Thus the first finger 16a is connected to the bottom of the envelope 10 whereas the second finger 16b has its end connected to the side wall of the envelope 10 and the third finger is connected to the upper wall of the envelope 10. This is best shown in Figure 2 and it is seen that the finger 16d is connected opposite finger 16b.

As the electron stream from the electron gun passes longitudinally through the envelope 10 toward the target 14, it is accelerated by an excitation source which will be later described and for this reason the spacing between the fingers 16 becomes progressively small. This is because as the velocity increases the transit time becomes less and less.

It is desired to excite the accelerator in a manner such that less than degrees phase difference occurs between adjoining fingers and this may be accomplished by multiphase feeding. An oscillator 18 produces an output which is fed to a coupling loop 19. The loop 19 passes through the wall of the envelope 10 and modulates the electron beam flowing between fingers 16a and 1617. A second coupling loop 21 passes through the wall of the envelope 10 and couples energy between the fingers 16b and 160. The loop 21 is connected to a phase shifting circuit 22 which receives an input from the oscillator 18.

An example of a suitable phase shifting circuit is shown in Figure 3. It comprises inductances L1 and L2 connected in series in the lines and capacitances C1 and C2 connected diagonally across the inductances L1 and L2. If L1 equals L2, and C1 equals C2, the phase shifter 22 will shift its input by 90 degrees and thus the energy fed between fingers 15b and 150 will be 90 degrees out of phase with the energy fed between fingers 16a and 16b.

The loops 19 and 21 couple energy into the electron beam through the space surrounding the fingers 16 rather than by inducing radio frequency voltages in the fingers 16. The phase difference between the coupling loops 19 and 21 determine the phase relationship of the modulation impressed on the beam. As previously shown, if multiphase excitation is used the shunt capacitance between adjoining fingers is increased, thus resulting in greater power developed with the same voltage. Since the energy fed to loops 19 and 21 has a phase relationship of less than 180 degrees, the phase relationship between adjoining fingers will be less than 180 degrees and therefore greater efficiency will be obtained. It is to be understood that the phase relationship between the coupling loops 19 and 21 may be 90 degrees, 60 degrees, or 120 degrees, for example.

It is seen that this invention provides means for obtaining greater efficiency in a linear accelerator by using multi-mode excitation.

Although it has been described with respect to a preferred embodiment thereof, it is not to be so limited as changes and modifications may be made therein which are 3 within the full intended scope of the invention, as defined by the appended claims.

I claim:

1. A multi-phase electron accelerator having a waveguide with an electron gun at one end and an oscillator situated external to said waveguide comprising, a plurality of members projecting from the inner wall of said waveguide axially removed from each other and with openings coaxial of the electron beam, a first loop supported in said waveguide between a first pair of adjacent projecting members and connected to the output of said oscillator, said first loop providing a first series of standing waves in said Waveguide, a second loop projecting between another pair of adjacent projecting members, said second loop providing a second series of standing waves in said waveguide with a space-phase of substantially 9-9 degrees with respect to said first series of standing waves, phase shift means connected between said second loop and the output of said oscillator, said phase shift means adjusted to provide a time-phase of substantially 90 degrees between the outputs of said first and second loops, whereby said first and second series of standing waves are space-phased and time-phased by substantially 90 degrees.

2. In an electron accelerator having a waveguide with an electron gun at one end and utilizing a means of oscillation comprising, a plurality of fingers extending radially from the internal surface of said waveguide and terminating somewhat beyond the axis of said waveguide, each of said fingers formed with a hole at the waveguide axis for the passage of electrons, said fingers spaced axially and angu'larly with respect to each other, a first means for dispensing radio-frequency energy located between the first and second of said fingers and connected to the output of said oscillation means, a second means for dispensing radio-frequency energy located between the secnd and third of said'fingers, a phase shift network connected between the output of said oscillator means and said second mean-s for dispensing radio-frequency energy,

and said phase shift network having a value that maintains said standing wave patterns with a time-space phase of degrees.

3. In an electron accelerator having a waveguide with an electron gun and utilizing an oscillator comprising, a plurality of fingers extending from the internal surface of said waveguide through the axis of the waveguide and formed with an opening at the axis, each of said fingers spaced axially and arranged ninety degrees with respect to the adjacent finger, a first means for injecting energy into said waveguide connected to the output of said oscillater and located between a first pair of fingers to provide a first standing wave pattern in said waveguide, said fingers alternately located at nodes of "said first standing wave, phase shift means connected to the output of said oscillater, a second means for injecting energy into said waveguide connected to the output of said phase shift network and located between a second pair of fingers to provide a second standing wave pattern in said waveguide, the alternate fingers located between the nodes of said first standing wave pattern being located at the nodes of said second standing Wave pattern, and said phase shift means adjusted to provide a ninety degree time phase between said standing wave patterns.

References Cited in the file of this patent UNITED STATES PATENTS 2,206,668 Hollmann July 2, 1940 2,222,902 Hahn Nov. 26, 1940 2,289,952 Zworykin July 14, 1942 2,323,613 Ludi July 6, 1943 2,368,329 Rosencrans Jan. 30, 1945 2,373,837 Linder Apr. 17, 1945 2,398,162 Sloan Apr. 9, 1946 2,464,349 Samuel Mar. 15, 1949 2,511,860 Lee June 20, 1950 2,653,271 Woodyard Sept. 22, 1953 

